1. Field of the Invention
The invention relates to anticontaminators for electron microscopes and especially to anticontaminators for side-entry transmission electron microscopes.
2. Description of Prior Art
Transmission electron microscopes (TEMs) are often supplied with an apparatus to minimize the deposition of contaminants such as carbon or ice which obscure specimen detail, distort the low energy region of X-ray spectra acquired from the specimen and make electron energy loss spectrometry impossible. The apparatus is generally known as an anticontaminator and comprises a dewar of liquid nitrogen mounted on the side of the TEM column with a conduction rod leading to a cold block onto which blades are mounted which surround the specimen. Presently available anticontaminators can be classified into two basic types.
Type 1 anticontaminators hold the liquid nitrogen coolant in an externally mounted, permanently sealed dewar and the conduction rod exits the microscope vacuum and dips into the liquid nitrogen coolant. The disadvantage of this arrangement is that the thermal efficiency of the anticontaminator is reduced by the additional heat flow into the short section of the cooling rod that is exposed to atmosphere in the space between the dewar and the microscope.
Type 2 anticontaminators hold the liquid nitrogen in a dewar which is continually evacuated by the microscope vacuum system. In this case the cooling rod does not exit the microscope vacuum and the thermal efficiency is therefore higher than the type 1 anticontaminators. However, because the Type 2 dewar uses the same vacuum system as the microscope the dewar must be emptied of liquid nitrogen whenever the microscope column is vented. This operation can be very tedious because vacuum seals must be broken in order to remove the dewar from the microscope to empty out the liquid nitrogen or alternatively the microscope operator must wait for the full reservoir of liquid nitrogen to be evaporated and the dewar heated to room temperature.
A further disadvantage shared by both Type 1 and Type 2 anticontaminators is that no provision is made for positioning the anticontaminator block during microscope operation. Positioning is desirable so that the entrance and exit holes for passage of the electron beam through the anticontaminator blades can made be as small as possible to achieve maximum degree of anticontamination and also to enable the blades to be removed completely from the specimen area if maximum specimen tilt becomes a priority.
Some manufacturers have incorporated contact sensors into their anticontaminators. At this time, however, these sensors have not made any allowance for the relatively high resistance of any ice layer which develops on the cold surface of the anticontaminator and they lack the sensitivity required for some applications.
Most users, therefore, would find it desirable to have an anticontaminator with its own self contained vacuum system, a high sensitivity contact sensor and an anticontaminator block with positioning capability.